When replacing existing turbine rotors, it is often necessary in the design of the new rotor to change the torsional frequency response. Various constraints, however, are often imposed requiring larger frequency windows with respect to a new rotor train. As a result, detuning of the rotor train may be required. Thus, material is frequently added to or removed from a rotor to achieve a certain frequency response and to move that frequency response to a desired frequency. The detuning process, however, may cause an increase or a reduction in the rotor LAN diameters. A reduction in the rotor LAN diameters is difficult to accommodate in existing diaphragms at the turbine installation site, especially in nuclear power plants because of the need for radiation protection. That is, upon reduction of the rotor LAN diameter, the stage sealing in the existing turbine would be lost unless measures are taken to restore the seal. A reduction in the rotor LAN diameter, for example, on the order of 2-4 inches, would result in a spacing of the existing packing ring segment from the rotor LAN, effectively forming an annular gap between the segment and the rotor LAN. The packing ring segment, of course, provides a seal between axial opposite sides of the stage and typically forms with the rotor LAN a labyrinth-type seal along its radially innermost surface. While the restoration of the stage seal can be accomplished by supplying a new stage diaphragm having an appropriately reduced diametrical bore, this is undesirable due to cost constraints and cycle time to manufacture and replace the old diaphragm with the new diaphragm. The existing stage diaphragm is therefore frequently modified under these conditions.
A typical modification to an existing turbine stage diaphragm includes building up weld material along the interior of the diaphragm bore. This technique is used where machinery can be brought to the installation site or the diaphragm can be transported to a facility capable of performing the modification. However, this technique is labor-intensive, contributes significantly to turbine outage time and costly.